The invention relates to an electrodeless low-pressure discharge lamp comprising:
a lamp vessel closed in a vacuumtight manner and comprising a neck-shaped end portion, which lamp vessel PA1 has an ionizable filling comprising a rare gas, and PA1 has at its end portion a tube which extends into the lamp vessel and which comprises a flared portion fused to the lamp vessel and connected thereto a cylindrical portion; PA1 a power coupler fastened to the lamp vessel in a detachable manner, which coupler comprises in the tube PA1 a core of soft magnetic material, PA1 an electric coil around the core, and PA1 a heat-conducting element in the core, which element extends to outside the lamp vessel and carries a flange there.
Such an electrodeless low-pressure discharge lamp is known from EP 0 456 289-A1 and also from U.S. Pat. No. 5,006,752.
The tube in the known lamp runs through substantially the entire lamp vessel length. The core with the coil lies in a zone situated at the end of the tube which lies inside the lamp vessel. A good transfer of the power into the ionizable filling is obtained thereby.
The known lamp is of a type which dissipates a comparatively high power, for example, approximately 80 to 90 W. Heat is generated during lamp operation, which heat should be removed in order to prevent the core assuming a temperature close to, or even above its Curie point. This is because the specific magnetic losses increase strongly with temperatures rising up to the Curie point, while the magnetic permeability decreases.
To counteract the risk of an excessive temperature of the core, the known lamp comprises a hollow pipe containing a liquid as the heat-conducting element. The liquid evaporates in a zone of the pipe situated in the core and the vapour condenses in a zone outside the lamp vessel. The condensation heat is transmitted to the surroundings by the flange. To ensure that the liquid flows to the zone in the core, also against the force of gravity if this zone should be above the flange, gauze is present in the pipe so as to provide a capillary structure which sucks the liquid upwards. The known tube provided with liquid and gauze has a very high heat-transporting capability, so that it is possible for the lamp to dissipate a comparatively high electric power with a comparatively high efficacy.
A disadvantage of the known lamp, however, is that the pipe is an expensive component.